The invention relates to a method for eliminating liquid components and fine-grained components from a sugar suspension, in which a certain fill quantity of the sugar suspension is spun in a centrifuge, with a certain quantity of water and/or steam added intermittently for a certain period of time, as well as to an apparatus for performing the method.
Such a method is used in the sugar industry particularly for removing the liquid component from the sugar suspension (also known as crystal suspension or magma) obtained in boiler apparatus. Centrifuges are used for this purpose, and the separation takes place in two phases:
After filling of the centrifuge with a certain predetermined quantity of sugar suspension, the spinning process begins, in which a sugar solution of lowest purity ("green runoff"), which contains all the substances incapable of crystallization, such as ash components, cellulose and the like, is precipitated out. This "green runoff" is used for further processing of the solution having the next lower purity;
Once this "green runoff" has been separated out, the so-called washing phase follows; that is, washing water is sprayed from nozzles onto the filter cake deposited at the circumference of the centrifuge. During this washing phase, any syrup residues still adhering to the sugar crystals are intended to be washed out; at the same time, the fine-grained components contained in the solution are dissolved and likewise washed out. Otherwise, the fine-grained components could cause plugging when the crystals are later filtered out with sieves. The liquid separated out during this phase is called the "washing runoff".
To increase the washing action it is also possible, instead of or in addition to spraying with water, to expose the filter cake to steam; in both cases, washing must be performed until such time as the syrup residues have been washed away from the crystal surface as completely as possible and throughout the entire thickness of the filter cake, i.e. the filter cake must be "washed through". On the other hand, however, prolonging of the washing process results in an unnecessary dissolving of additional sugar, which would have to be subsequently recrystallized, a process that again requires thermal energy.
The fact that the composition of the crystal suspension may undergo major fluctuations under some circumstances, particularly in terms of the crystal sizes and especially the fine-grained components, makes it impossible to arrive at fixed values for optimizing the centrifuging and washing process, although, as explained above, such fixed values would, on the one hand, assure the completest possible washing and, on the other hand, would prevent unnecessary prolongation of the process, with the attendant poorer overall results in terms of cycle time and energy consumption.
If the syrup components in the green runoff phase drain off quickly, then it can be concluded that the fine-grained component is proportionately small, and consequently the water quantity in the washing phase can also be kept relatively small. If the outflow of syrup components in the green runoff phase is relatively slow, then it can be concluded that the fine-grained component is proportionately very large and the permeability of the filter cake is low; consequently the water quantity during the washing phase must be increased, or the fill quantity of the centrifuge must be reduced in the next cycle. Otherwise, because of the reduced permeability of the filter cake, a certain backup of fluid can occur in various layers, and this in turn again leads to an undesirable partial dissolution of crystals.
This makes it clear that a plurality of parameters, such as the fill quantity of the centrifuge having the crystal suspension, the quantity of water and/or water vapor used for washing, the beginning and end of the centrifuging process, and the beginning and end of the washing process determine the quality of the outcome of the method both individually, and in their functional dependency on one another.
Previous attempts to define these process parameters as optimally as possible, in the sense described above, have been limited to monitoring the various end products, that is, the remaining crystals or the washing runoff, by taking laboratory samples, for instance by refraction measurements. Such random sampling is very time-consuming and labor-intensive; the results are not available immediately; and the sampling is of necessity highly inaccurate.
Hence this known approach to determining the process parameters can merely serve to prevent the gravest control errors.